Electrochemical behavior of biocatalytical composite based on heme-proteins, didodecyldimethylammonium bromide and room-temperature ionic liquid

A novel biocompatible composite film based on a water-insoluble surfactant, didodecyldimethylammonium bromide (DDAB), and a hydrophobic room-temperature ionic liquid (RTIL), 1-hexyl-3-methyl-imidazolium hexafluorophosphate (HIMIMPF6), for the immobilization of biocatalytical proteins was reported. Differential scanning calorimetry (DSC) showed that the DDAB–HIMIMPF6 composite film has higher thermal stability than the DDAB film alone. SEM images indicated that different microstructures existed between the DDAB film and the composite film, indicating the interaction between DDAB and RTILs. This composite can be used as the immobilization matrix of proteins and other biomacromolecules. Heme-proteins, including hemoglobin (Hb), myoglobin (Mb) and horseradish peroxidase (HRP), were used as model proteins for studying the electrochemical behaviors of the resulting biocatalytical composite films. In the case of Hb, a pair of well-defined quasi-reversible redox peaks was obtained when the composite film containing Hb was modified on a glassy carbon electrode. The formal potential (E°′), the surface coverage (Γ*) and the electron transfer rate constant (ks) were calculated as −0.308 V, 1.32 × 10−11 mol cm−2 and 11.642 s−1, respectively. While, these parameters for Hb on DDAB films alone were −0.309 V, 7.20 × 10−12 mol cm−2 and 2.748 s−1, respectively. Therefore, the composite are more suitable for the direct electron transfer between Hb than DDAB alone. The native conformation and bioactivity of Hb adsorbed on the composite film was proved to be maintained, reflected by the unchanged ultraviolet–visible (UV–vis) as well as the catalytic activity toward hydrogen peroxide (H2O2) and nitric oxide (NO) compared with the free Hb molecules. Furthermore, Hb on the composite film are more sensitive for the detection of hydrogen peroxide (H2O2) and nitric oxide (NO) than that on DDAB film alone. The linear range of H2O2 on Hb/DDAB–RTILs/GC electrode is from 0.5 to 57.5 μM with linear regression equations I(μA) = 0.149 + 0.00904C(μM), while, the linear range of H2O2 on Hb/DDAB/GC electrode is from 0.5 to 57.5 μM with linear regression equations I(μA) = 0.0938 + 0.00553C(μM). For NO, its linear range on Hb/DDAB–RTILs/GC electrode is from 1.8 to 21.6 μM with linear regression equations I(μA) = 0.0937 + 0.0232C(μM). But its linear range on Hb/DDAB/GC electrode is from 1.8 to 21.6 μM with linear regression equations I(μA) = 0.0285 + 0.0167C(μM). Similar results were observed for Mb and HRP in the DDAB–HIMIMPF6 composite film.